Tiles inside your blood tubes
Your heart pumps blood through tubes called blood vessels. Some are as thick as a pencil. Others are thinner than a hair.
All of them are lined on the inside by endothelial cells. These cells are very thin and flat. They fit together edge to edge, like tiles on a bathroom floor.
Wow!
In the tiniest blood tubes, called capillaries, the whole wall is just one thin layer of endothelial cells. That is how oxygen and food can slip through to your other cells.